On 9/11/07, Bruce Momjian <[EMAIL PROTECTED]> wrote:
> Heikki Linnakangas wrote:
> >
> > Pruning does generate a WAL record at the moment. Maybe you could do
> > some kind of a quick pruning without a WAL record. Like just modify the
> > ctid of the oldest dead tuple in the chain, or the redirecting line
> > pointer if there is one, to point to the latest live tuple, without
> > removing the dead tuples or the line pointers.
> I am wondering what you even mean by removing the dead tuples when
> pruning.  I thought only defragmentation removed tuples.
Pruning removes intermediate dead tuples by marking their line pointers
~LP_USED and redirecting the root line pointer to the first
tuple in the chain. OTOH defragmentation moves tuples around to repair
fragmentation. IOW defragmentation is nothing but calling
PageRepairFragmentation on the page.

For example, if we have a HOT chain of 5 tuples:

1 --> 2 --> 3 --> 4 --> 5

of which, 1, 2 and 3 are DEAD, 4 is RECENTLY_DEAD and 5 is LIVE

At the end of pruning:

- line pointer of 1 is redirected to 4
- line pointers of 2 and 3 are marked ~LP_USED
- the offset of 4 and 5 is unchanged

At the end of defragmentation:

- 4 and 5 are moved to the end of the page to create a larger
contiguous free space in the page.


Pavan Deolasee
EnterpriseDB     http://www.enterprisedb.com

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